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This is a reland of 9b785217 with test
fixes. The failing unit tests weren't being run when I first landed
this, but were enabled afterwards and started breaking, so I rolled
back.

Specifically, I PostTask'd (un)registering with the
NetworkConnectionTracker to the right threads, and added a
PrivetTrafficDetector::Stop method to clean up the registration and
call it from PrivetNotificationService.

Original change's description:
> Migrate PrivetTrafficDetector to use NetworkConnectionTracker.
>
> This migrates PrivetTrafficDetector from using
> net::NetworkChangeNotifier to content::NetworkConnectionTracker, which
> works with the network service enabled.
>
> Bug: 859134
> Change-Id: I06f2ecbe142bbed9a8772f69ab109b0c0dc32581
> Reviewed-on: https://chromium-review.googlesource.com/1123681
> Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#574700}

Bug: 859134
Change-Id: I19bc5bf0abdbd41cfbd3da1c0881f122adca6c2b
Reviewed-on: https://chromium-review.googlesource.com/1137411Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576278}